With the progress in digital processing and communication technology, it is becoming quite common to digitize signals produced by analog sources for further processing in the digital domain, for storage in digital form, or for transmission through digital media. However, in the digital domain, there are different sampling rates used depending on the required quality and the available bandwidth. Thus, it has become important to have simple digital interfacing between different equipment and media.
The concept of sample rate conversion ("SRC") is well known to those who work with digital signals for multimedia applications. A conventional approach to the application of SRC is to first digitize some analog signals at a constant sample rate using an analog-to-digital converter ("ADC"). The digital signals are then applied to the SRC so that the converted signals will have a sample rate suitable for the intended purposes and applications. The SRC may have the following applications, as an example:
In order to transmit high-quality ("HQ") audio signals through a communications channel, e.g. phone line, with bandwidth 4 kHz, it is required to down-convert the source signals' sample rate to 8 kHz for the transmission, since the HQ signals have typically been sampled at a sample rate of 44.1 kHz or 48 kHz.
Also, to digitally mix signals from CD-ROM with speech annotation, it is necessary to convert their sample rates to the same sample rate for the mixing, since the CD-ROM has a sample rate of 44.1 kHz, while speech typically has a sample rate of 8 kHz. While it is possible to convert both signals into analog for mixing in the analog domain, it has been quite expensive in the analog domain due to the need for DAC and ADC, not to mention the attendant distortion problems.
Further, to digitally store the digitized speech signals on a PC for later processing, SRC is also needed, due to the difference in sample rates.
FIG. 1 illustrates a conventional SRC for HQ audio signals. Such a system may include a mixer or microphone 110 for receiving analog source signals 100 such as music or speech, the received signals are then converted into an oversampled digital form by a sigma-delta analog/digital converter (ADC) 120. The digital signals are then applied a decimation filter 130 for converting its sample rate to, e.g. 44.1 kHz, before they are applied to the SRC 140 to down convert its sample rate to a range between 4 and 44.1 kHz. The resulting signals 145 can then be transmitted through a telephone link 150, stored in a permanent memory, or storage disk 155 or mixed with other source signals 160.
But the conventional system is quite costly both in memory and computation cycle requirements in order to be able to convert from 44.1 down to 4 or 8 kHz, since this is a down conversion factor of more than 11 and 5, respectively. Since the required resources for the SRC both in memory and computations are proportional to the down sample ratio of the SRC, it is thus desirable to have a much more efficient implementation than the conventional approach.